HIGH FLUX DENSITY ROTATIONAL CORE LOSS MEASUREMENTS

Energy conversion processes involve losses. Specifically core losses, which are a result of the magnetization process in cored electrical energy conversion and storage devices. The cores are made of soft ferromagnetic materials that are easily magnetized and demagnetized. These soft magnetic cores, allow a reduction in size, higher energy storage density, and a reduction in magnetizing current, when compared to non-cored devices. The characterization of soft ferromagnetic materials is traditionally done under unidirectional pulsating fields, which is sufficient for single-phase transformers and inductors, where the cores are under pulsating fields. However, T-joints of three phase transformers and teeth-roots of rotating machine stator cores are exposed to two-dimensional rotational fields of higher core loss. Pulsating measurements are therefore insufficient in the characterization of soft ferromagnetic materials used in rotating electrical machines or in three phase transformers. In two-dimensional fields, the magnetization direction changes with time, tracing a flux density locus. This requires the measurement of tangential magnetic field and flux density components, hence the associated loss. This study proposes a two-dimensional rotational core loss tester for high flux density measurements up to about 2 T, at 60 Hz. Its frequency measurement range is from 60 Hz to 1 kHz. The initial sizing was done analytically, then implemented in three-dimensional finite element analysis, prototyped and experiments performed to verify its capability. It was validated by testing two 0.35 mm and 0.65 mm thick samples. Very high flux densities in the range of 2 T at 60 Hz were achieved in both samples. For the thinner sample, flux densities of 1.8 T and 1.6 T were measured at 400 Hz and 1 kHz, respectively, while for the thicker one, the range reduced to 1.7 T and 1.4 T, at 400 Hz and 1 kHz, respectively. The magnetizer also reproduced non-sinusoidal flux density waveforms, for flux densities less than or equal to 1.0 T, without any waveform control. The proposed rotational core loss setup will find application in the characterization of electrical steels, and generation of pulsating and rotational core loss data. This data can then be applied in core loss models, uprating of megawatt (MW) rated machines, transient and hotspots analysis, and in the design of higher power density machines, such as high-speed machines